The invention relates to a circuit that, while useful in other devices, is useful primarily in complementary metal oxide semiconductor (CMOS) circuits. Since CMOS circuits are commonly employed in battery powered equipment it is desirable that a battery condition monitor be incorporated to indicate when the battery voltage has dropped below a particular level which indicates that replacement or recharging is in order. Also, when an A-C line rectified power supply is employed, its output can decline due to the loss of one or a few cycles of excitation. The lowered supply voltage state is commonly called brownout because the next stage, or blackout, will occur when the voltage has fallen to a point where the circuits will not operate. Typically, a switching circuit is employed and its switching level set to sense a brownout level prior to the blackout level. Most of the prior art battery condition indicators show when an arbitrary voltage level is reached and this level is chosen to be sufficiently above the blackout level that when all manufacturing tolerances are considered the circuit performance is reliable in all cases. This results in a relatively high switching level. It would be more desirable to set the switching level lower and in response to the circuit function itself. If the switching level is set to be just a small increment over that level at which the circuits function, manufacturing tolerances will have no effect and a true brownout level is indicated. This would be equivalent to an adaptive reference level keyed to circuit performance rather than an arbitrary value.
Another consideration relates to fluctuations in battery or supply voltage which manifest themselves as noise on the supply lines. Such noise can result from the operation of associated circuitry or the switching on and off of other devices connected to the same supply. It is desirable that the brownout detector be as immune to such noise as possible and yet be able to respond when a true dropout has occurred.
Since the devices described here may relate to battery operation, it is also desirable that current consumption be minimized To this end, it is desirable that the brownout detector itself be operated at the lowest possible current. Accordingly a shut-off feature is desirable wherein the detector can be turned off or reduced to a state of insignificant current consumption when it is known that no indication is needed. This would be the case, for example, just after battery replacement or recharging. Here it is known that the equipment will operate normally for at least a known period of time. Or, a mask option may be used to manufacture a chip with the brownout detector permanently disabled or enabled, at the customer's choice.
U.S. Pat. No. 4,701,639, issued Oct. 20, 1987, to Silvo Stanojevic, and is assigned to the assignee of the present invention. It discloses a threshold detector circuit that is designed to monitor a power supply voltage and produce an output signal when the voltage drops below the threshold. In this circuit, bipolar elements are employed and a well-known bandgap temperature compensated voltage reference provides the desired threshold. Hysteresis is added to the bandgap circuit to improve noise immunity.
Matsuura patent 4,024,415, issued on May 17, 1977, and discloses a CMOS battery voltage detecting circuit. The threshold voltages of a complementary pair of transistors provide the critical value of a voltage detector. However, the circuit shown suffers from the fact that there is no way that current can flow through the complementary pair in the FIG. 2 embodiment, and erratic performance would result. In the FIG. 3 embodiment, resistivity operated field effect transistors (FETs) are employed to force separate and independent currents through the complementary pair elements.
The teaching in the above two patents is incorporated herein by reference.